Page 109 - Boiler plant and distribution system optimization manual

94 Boiler Plant and Distribution System Optimization Manual
• Check control elements for smooth accu-
rate operation. Correct unnecessary hunting
caused by improperly adjusted regulators
and automatic master controllers.
• Inspect all fuel valves to verify proper move-
ment, clean and repair as necessary.
FURNACE
• The firesides should be clean, check for soot-
blower cleaning efficiency. Consider period-
ic water-washing if firesides are not being
kept clean by normal soot-blowing.
• Inspect and repair internal baffling. Defec-
tive baffling allows hot combustion gases to
escape without giving up heat causing high
stack temperatures. A traverse of the breach- every point in the flame a balance exists between
ing with a temperature indicator may point
out local hot spots behind baffle defects.
Once hot spots are identified, the defects can
be corrected.
• Repair any casing leaks and any cracked or
missing refractory.
• Clean furnace-viewing ports and make sure
that burner throat, furnace walls and leading
convection passes are visible. Being able to
see the condition of the flame, burner, refrac- for different applications Figures 9.10 through
tory zone and furnace is essential to detect- Figures 9.16 The topic of NO control figures
ing and correcting problems.
FUNDAMENTAL COMBUSTION
CHARACTERISTICS
The chemistry and physics of combustion
very intricate and very difficult to define and de-
scribe. Combustion is notoriously elaborate and
complex field of study. An attempt will now be
made to put a simple face on the subject.
One of the main challenges faced by boiler
and burner manufacturers is to design a compact
and stable combustion system. Compactness, for
lower first cost, and stability for trouble free and
efficient operations. It has been well known for
many years that a swirling motion of a burner
flame will cause recirculation patterns which pro-
duce longer residence time, better air fuel mixing
and stable flame conditions. Figure 9.6 is an ex-
ample of the swirl pattern developed in register
type burners.
Basic factors that must be considered when
working with burners is burning velocity as influ-
enced the air/fuel ratio and the use to toroids in
the combustion zone. Figure 9.7 shows how burn-
ing velocity changes as the air/fuel ratio varies
from stoichiometric conditions.
Figure 9.8 shows the use of toroids to sta-
bilize the combustion zone. The recirculation
patterns developed in the toroid zones have a
great influence on combustion behavior. Swirl im-
proves flame stability by forming toroidal recir-
culation zones that recirculate heat and unburned
fuel constituents back to the base of the flame. At
the velocity of flame propagation and the stream
of incoming air and fuel. This also includes off
stoichiometric mixtures of forming gases made
up of partially burned mixtures of air and fuel
and at varying levels of combustion activity. The
trick is to get a complete but stable burn with no
fuel energy wasted and the least amount of excess
air possible. Figure 9.9 is a sketch of the final re-
sult of such a design.
Various flame types have been developed
x
into these various flame types as shown in figure
Figures 9.17, the peak in NO production belongs
x
to the ball shaped flame which has extensive re-
circulation. It can be seen that as the surface area
of the flame increases relative to volume, the tem-
perature and NO production falls off.
x
FLAME APPEARANCE
The flame is the heart of the combustion
process, if it isn’t right you will have a serious
challenge tuning-up a boiler.
The appearance of a boilers flame offers a
good preliminary indication of combustion con-
ditions. It is difficult to generalize the character-